1. Field of the Invention
The present invention relates to AC power sources and, more particularly, to a solid state AC power source which is characterized by the absence of an output transformer.
2. Description of the Prior Art
In prior art solid state AC power sources, in order to eliminate the type of failure of output transistors, known as "second-breakdown", power is first generated at a relatively low voltage by means of the power amplifier section, so that the transistors are below their second-breakdown failure region. This region varies with transistor type. Generally in the prior art the maximum voltage seen by the transistors is kept below 150-200 volts which is below the second breakdown failure region for most types of transistors. Thus, in the prior power source the AC power, which is generated by the power amplifiers, is always below 125v-130v rms which is usually required for most power sources. The lower than required output voltage is increased by stepping up the voltage of the generated power by means of an output step-up transformer.
As is appreciated by those familiar with the art the presence of the output transformer is undesirable for many reasons. Its presence adds significant bulk, weight and cost. This is due to the fact that the transformer requires a large heavy core of low-loss material, relatively few turns, and carefully controlled winding geometry, in order to enable it to withstand maximum voltage and power at low frequencies as well as operate at low loss at high power frequencies. In addition, and possibly more important, the need for an output transformer forces serious compromises in electrical output characteristics.
In the prior art power sources, the feedback signal is provided to the power amplifier from the transformer primary, which results in loss of regulation, distortion and transient response to the secondary winding to which the load, which is typically a non-linear load, often with a very low power factor, is connected. The load is actually a part of the circuit and affects the transformer output voltage. Theoretically, it would be desirable to stabilize the power source by providing feedback from the transformer secondary winding to which the load is actually connected, rather than from the primary winding. Such feedback, however, is not possible because of variable transformer phase shift with operating frequency and with load reactance variations. Thus, the feedback from the primary, rather than from the secondary, results in regulation loss. Due to the high phase shifts caused by the transformer leakage inductance and load capacitance most prior art power sources, using primary winding feedback, do not operate with any degree of satisfactory stability with loads of very low power factors.
In some power sources with an output transformer, open loop compensation is attempted. However, such compensation is itself feedback which often causes the circuit to oscillate. Practically all of these problems can be eliminated by the elimination of the output transformer and by providing feedback directly from the output terminals to which the non-linear load is actually connected.